On 12/09/2015 05:35 PM, Ben Boeckel wrote:
> So some behavior I was unaware of until today came up:
>
> set(var ON)
> option(var "description" OFF)
> message("var: ${var}")
>
> outputs "OFF" for the first configure and "ON" for subsequent
> configures. This is because set(var CACHE) does unset(var) *if* the
> cache was touched. This is not done on the second time around since it
> is already in the cache.
That is a long-standing subtlety introduced without discussion, review,
or tests here:
BUG: change in how set cache overrides local definitions. Should mainly be a
NOP change for most cases
https://cmake.org/gitweb?p=cmake.git;a=commitdiff;h=f52d37c2
IIRC there was confusion at the time in the case of
set(var 1)
set(var 2 CACHE STRING ...)
message("${var}") # prints "1" before the above change.
> I think a policy to remove the unset(var) behavior should be added since
> the current behavior means that clean builds can be wildly different
> than incremental builds.
One reason a policy has not been introduced for this before is that
producing a warning for the policy may be very verbose unless it is
delayed until variable dereference, but the latter would be a huge
performance hit to check. Still, I think things would be better off
in the long run with some policy for it.
> Related, I have a branch on the stage (update-variable-docs) which
> attempts to clarify some darker corners of the set() command and the
> *VARIABLES directory properties.
The "LOCAL_VARIABLES" change uses "SCOPE_VARIABLES" in some places.
The release note should only cover the new feature.
-Brad
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